Adult learners often find it difficult to acquire a high level of proficiency in a second language (L2). The literature on L2 learning is rich with evidence and debate concerning the source of constraints that potentially challenge complete acquisition at each level of language processing. Despite the documented constraints, some learners manage to successfully achieve native-like performance in the L2, but the evidence on what it takes for a learner to be successful or for a learning context to be enabling is mixed. Having a high level of cognitive resources or being immersed in the L2 context all appear to contribute to positive outcomes, but not uniquely. In the planned research we take direction from recent findings on proficient bilinguals to test a new hypothesis about second language learning. The hypothesis is that successful L2 learners are individuals who are able to tolerate change to the native language. That change may involve processing costs that initially slow the native language and make native language performance more error prone, make learners less sensitive to some features of the native language, and that open the native language to the influences of the L2. High levels of cognitive resources and immersion in the L2 may enhance this process but what is hypothesized to be fundamental is change to the first language (L1) that functionally allows the L2 to develop as part of the language system. We review the background on bilingualism on which this hypothesis is based, present pilot data that reveal costs to language processing during early stages of L2 learning, and then consider evidence from the literature on learning and memory that suggests that initial costs to learning may result in more successful later outcomes. We then outline a program of research that investigates changes to the native language that occur during the course of adult L2 learning. We will examine learners at different levels of proficiency, in typical classroom learning contexts and under conditions of language immersion. We contrast changes to the native language at the level of the lexicon, the grammar, and the phonology, comparing comprehension and production, using behavioral measures of speed and accuracy, eye tracking measures during reading, and electrophysiological measures that examine the earliest time course of language processing. Although the focus in the planned research is on the L1, all experiments also assess performance in the L2 to consider the relationship between L1 change and L2 performance. A specific question is whether there is a relationship between L1 change during L2 learning and the degree of L1 transfer, a mechanism that has been identified as an important feature of initial L2 learning. The new hypothesis leads to the counterintuitive prediction that individuals who are able to tolerate change to the native language may be less likely to depend as heavily on L1 transfer as individuals who maintain the autonomy of the L1.